The present disclosure relates to continuous, unsupported, microporous membranes having two or more distinct, but controlled pore sizes and to processes of making and using same, more particularly to unsupported microporous membranes made from a first dope and at least one additional dope being applied directly to one another prior to the at least two dopes being quenched and to apparatus for manufacturing and processes for making such membrane.
Microporous phase inversion membranes are well known in the art. Microporous phase inversion membranes are porous solids which contain microporous interconnecting passages that extend from one surface to the other. These passages provide tortuous tunnels or paths through which the liquid which is being filtered must pass. The particles contained in the liquid passing through a microporous phase inversion membrane become trapped on or in the membrane structure effecting filtration. The particles in the liquid that are larger than the pores are either prevented from entering the membrane or are trapped within the membrane pores and some particles that are smaller than the pores are also trapped or absorbed into the membrane pore structure within the pore tortuous path. The liquid and some particles smaller than the pores of the membrane pass through. Microporous phase inversion membranes have the ability to retain particles in the size range of from about 0.01 or smaller to about 10.0 microns or larger.
Many important micron and submicron size particles can be separated using microporous membranes. For example, red blood cells are about eight (8) microns in diameter, platelets are about two (2) microns in diameter and bacteria and yeast are about 0.5 microns or smaller in diameter. It is possible to remove bacteria from water by passing the water through a microporous membrane having a pore size smaller than the bacteria. Similarly, a microporous membrane can remove invisible suspended particles from water used in the manufacture of integrated circuits in the electronics industry.
Microporous membranes are characterized by bubble point tests, which involve measuring the pressure to force either the first air bubble out of a fully wetted phase inversion membrane (the initial Bubble Point, or xe2x80x9cIBPxe2x80x9d), and the higher pressure which forces air out of the majority of pores all over the phase inversion membrane (foam-all-over-point or xe2x80x9cFAOPxe2x80x9d). The procedures for conducting initial bubble point and FAOP tests are discussed in U.S. Pat. No. 4,645,602 issued Feb. 24, 1987, the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure. The procedure for the initial bubble point test and the more common Mean Flow Pore tests are explained in detail, for example, in ASTM F316-70 and ANS/ASTM F316-70 (Reapproved 1976) which are incorporated herein by reference to the extent not inconsistent with the present disclosure. The bubble point values for microporous phase inversion membranes are generally in the range of about two (2) to about one hundred (100) psig, depending on the pore size and the wetting fluid.
An additional method which describes a pore measurement technique is ASTM E1294 89 which describes a method for determining pore size by clearing fluid from the pores of the membrane and measuring the resulting flow. This method is used to measure mean flow pore but is similar to the method of Forward Flow Bubble Point in that the wet portion of the test uses a similar protocol.
The Forward Flow Bubble Point (FFBP) test is described in U.S. Pat. No. 4,341,480 by Pall et. al., the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure. This patent discloses how the FFBP can be used to distinguish a symmetric membrane from an asymmetric membrane. The FFBP curve is generated by saturating the membrane with fluid and subjecting one side to a rising air pressure while measuring air flow on the downstream side. For a single layer symmetric membrane with a well defined pore size, a plot of air flow versus pressure remains flat but slightly above zero due to diffusion through the liquid in the membrane. When the pressure reaches a point when it can overcome the surface tension of the fluid in the pore, the air will push the liquid out of the pore and air will flow through the pore in bulk (bulk flow). This pressure point is a function of the surface tension of the liquid and the radius of the pore as defined by the equation of Young and Laplace (see Physical Chemistry of Surfaces by Arthur Adamson, Wiley Press). When the pores all have essentially the same size, this event occurs simultaneously and is characterized by a transition of the flow versus pressure curve from horizontal (when diffusion flow is predominant) to vertical (where bulk flow is dominant), this type of FFBP characteristic is shown in FIG. 9. FIG. 9 also demonstrates that the FFBP characteristics for a single layer symmetric membrane are identical regardless of membrane orientation.
On the other hand, asymmetric membranes are characterized by a gradual change in pore size throughout the thickness and exhibit a different FFBP curve, when tested with the large pore size surface facing up stream against the applied air pressure. Since the pore size is gradually changing throughout the thickness depth, the pressure required to push fluid down the pores rises gradually and the resulting FFBP curve has a rising slope until the final bubble point is reached and bulk flow occurs. While an asymmetric membrane might be retentive, the above response is indistinguishable from an asymmetric membrane with defects, where certain pores are significantly larger than the remaining pores and exhibit bulk flow at lower pressure. The FFBP response of this type of membrane also exhibits a rising slope when flow versus pressure is plotted.
U.S. Pat. No. 3,876,738, the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure, describes a process for preparing microporous membranes by quenching a solution of a film-forming polymer in a non-solvent system for the polymer. U.S. Pat. No. 4,340,479, the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure, generally describes the preparation of skinless microporous polyamide membranes by casting a polyamide resin solution onto a substrate and quenching the resulting thin film of polyamide.
There is an extensive body of knowledge concerning the production of multiple layer films using pre-metered coating technology, such as, for example, slot dies, as taught by. This prior art deals with the extrusion of films that are essentially impermeable. This prior art also discusses manufacture of both photographic film and films used in the packaging industry (e.g. food packaging). Some examples of patents, each of which are herein incorporated by reference to the extent not inconsistent with the present disclosure, disclosing multilayer films are listed in the table below:
At least some of the above prior art teaches the use of pre-metered dies to apply coatings in the production of essentially non-porous films. Discussion of pre-metered dies can be found in two Troller Schwiezer Engineering (TSE) publications, xe2x80x9cConcepts and Criteria for Die Designxe2x80x9d and xe2x80x9cPrecision Coating: Pre-metered and Simultaneous Multilayer Technologies,xe2x80x9d which are available from TSE upon request. Pre-metered coating methods comprise slot, extrusion, slide and curtain coating. Pre-metered coating processes are characterized by the fact that the down-web thickness of the coated layer is solely determined by the ratio of volumetric flow rate/width of fluid pumped into the die to the speed of the web. A discussion of multiple slot dies are also presented in a Master Thesis written by Shawn David Taylor titled Two-Layer Slot Coating: Study Of Die Geometry And Interfacial Region at McMaster University, dated July 1997, the disclosure of which is hereby incorporated by reference to the extent not inconsistent with the present disclosure.
Other art involves the manufacture of microporous membranes by other techniques. Grandine provides the first practical disclosure of the manufacture of PVDF membrane. The Grandine patent (U.S. Pat. No. 4,203,847) discloses, although does not claim, that thermal manipulation of the dope will lead to a change in pore size of the resulting membrane. Surprisingly, given that nylon is a very different polymer that is dissolved in ionic organic acids rather than an organic ketone, it experiences a similar phenomenon. Grandine did not suggest a mechanism for this phenomenon to indicate that it might be general for polymers used to make membranes.
Subsequent patents relating to PVDF disclose methods for making asymmetric PVDF membrane. The Wang patent (U.S. Pat. No. 5,834,107) discloses a variety of methods to manufacture asymmetric membrane. Other patents that are related to asymmetric structure and which are cited in the Wang patent are Costar (WO 93/22034), Sasaki (U.S. Pat. No. 4,933,081), Wrasidlo (U.S. Pat. Nos. 4,629,563 and 4,774,039), and Zepf (U.S. Pat. Nos. 5,188,734 and 5,171,445).
Asymmetric membrane prior art does not disclose, suggest or teach independent control of the properties of each zone (such as thickness or pore size) nor the formation of distinct layers or two distinct polymer dopes.
Other prior art is the use of thermal manipulation to create distinct zones of controlled pore size with nylon membrane by Meyering et al. as disclosed in (PCT publication WO 99/47246, the disclosure of which is incorporated herein by reference to the extent not inconsistent with the present disclosure) applying two layers of dope against opposite sides of a support scrim after the scrim was filled with a first dope. In some applications, especially pleated cartridge filters, Nylon is an intrinsically weak material which requires the use of a scrim to function in particular applications effectively, but unreinforced or unsupported nylon is used in other applications. The presence of the reinforcing or supporting scrim requires multiple dies, one to provide dope within and to fill the scrim for the middle membrane zone and the other two dies to apply the dope for the outer two membrane zones. In order for the Meyering process to effectively operate, a centrally positioned porous support which remains with the finished membrane is clearly required regardless of whether or not the membrane polymer requires the support for a particular end use application.
Additional prior art is Degen (U.S. Pat. No. 5,500,167) which also claims a supported membrane with a porous nonwoven fibrous support wherein the two zones of the membrane are divided into zones of different pore sizes. In that case, a second dope layer to form a second zone is applied to a first dope layer in a secondary, sequential operation with the scrim partially outboard of the two finished zones.
Additional prior art is Holzki U.S. Pat. No. 5,620,790 which describes a multilayer membrane applied with a doctor blade but is subject to the restriction that the viscosity of the first layer in the polymer solution form must be greater or equal to the viscosity of subsequent layers. This viscosity restriction require either solids manipulations or the addition of viscosity enhancing agents to control membrane formation. Adulteration of the polymer solution in this manner is less desirable than a technique which is not sensitive to viscosity differences between the layers.
Tkacik U.S. Pat. No. 5,228,994 mentions in passing, although does not claim, that membranes may be coextruded in a multilayer sequence (column 3 line 46-50) prior to either layer being phase inverted. However, the examples only reference methods that are suitable for coating a polymer solution layer to a already formed substrate, which is the main topic of the patent. This patent does not disclose a method to produce a multilayer membrane where neither layer has previously been subjected to phase inversion.
Steadly U.S. Pat. No. 4,770,777 deals with skinned multi-layer membranes made by a post-metering process.
PCT publication WO 01/89673 A2 to Kools, the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure, appears to disclose xe2x80x98co-castingxe2x80x99 as a method to make multilayer PVDF membrane. Having as its salient point post-metering coating apparatus which apparently results in high interfacial shear turbulence which results in an asymmetric transition zone, having a different pore size from either of the two adjacent layers, in the interface zone. It is believed that the Kools structure, as disclosed, will result in the undesirable FFBP as discussed below.
All of these preceding methods disclose the use of post-metering processes, which utilize apparatus, such as, for example, casting knives or doctor blades to produce membranes, whether they are single or multi-layer. The casting knife is a post-metered approach where the thickness of the applied polymer solution is controlled by the application of a device, such as, for example, a spreading bar in contact with the top surface layer of the coated material as it is applied to the substrate. These methods suffer from the limitation that they create an asymmetric region at the interface presently believed due to the shear action of post-metered coating apparatus.
Another approach to joining two different membrane zones together so as to produce a multilayer membrane is wet laminating wherein membrane precursors that have been cast and quenched but not dried are joined under mild pressure and then dried together. When the pore sizes are different from each other and both layers are symmetric, the asymmetric transition is eliminated and a desirable FFBP curve response is generated, as shown in FIG. 8. However, wet lamination is prone to delamination, which can be a particular concern if the membrane is back-flushed. As a practical matter, laminated multilayer membranes tend to be thicker than single zone membranes since each zone is an independently, individually prepared membrane which included being quenched prior to being laminated together to form the multilayer membranes. These prior art membranes are clearly relatively thick, as each zone of the laminated multilayer membrane must be individually sufficiently thick in order to survive the membrane manufacturing process and then be joined with at least one other individual sufficiently thick membrane, individually and separately prepared, to form a multilayer laminated membrane.
Prior art on pre-metered application technology, which includes the use of slot dies, generally does not deal with and is not believed to have been applied to the manufacture of microporous membranes with the exception of the Meyering et al. disclosure mentioned above.
Thus, there is a need for unsupported or scrimless, multilayer polymeric microporous membrane having at least two independent and distinct pore size layers progressing through the thickness of the membrane, each layer being continuously joined to its adjacent layer throughout the membrane structure. Such a multilayer membrane should eliminate the need for reinforcing or supporting scrim while realizing the advantages of multilayer filtration control. Such a scrimless multilayer membrane should have at least two separate layers that are continuously joined by the molecular entanglement that occurs in the liquid between the two dope layer prior to phase inversion but with a sharp pore size transition between the two layers. Such a multilayer scrimless membrane should be preferably as thin as prior art single layer membranes and thinner than prior art laminated multilayer membranes. Such a membrane should exhibit a FFBP curve such that it can be distinguished from a defective membrane.
The present disclosure is directed to unsupported (without an integral reinforcing or supporting porous support) multilayer microporous membrane, apparatuses and processes for the manufacture thereof. The unsupported membrane may be substantially simultaneously formed into multiple (two or more) discrete layers, each with, presently preferably, a different but controlled pore size. The unsupported membrane may also comprise multiple (two or more) discrete layers each with, presently preferably, a different but controlled pore size, with a distinct change in pore size at the interface between each of the layers that does not exhibit locally asymmetric pore size distributions, such that the resulting membrane exhibits a Type I Forward Flow Bubble Point (FFBP) curve response, as illustrated in FIGS. 16a and 16c, and demonstrated in FIG. 12 and discussed below.
Layers of dope that eventually form the layers are applied directly to one another prior to the membrane quench such that interfacial turbulence and gross mixing between adjacent layers are avoided, maintaining distinct pore sizes within the separate layers but where the separate layers are integrally joined at each interface. A multilayer membrane structure results from the process step of applying the individual dopes or polymer solutions that form each of the layers sequentially onto one another, the resulting multilayer liquid coating, subsequent to being subjected to a process step that induces phase inversion that forms the distinctly sized pores in each layer, with each porous layer being physically bonded to its adjacent porous layer by polymer intermingling, at a molecular scale, at the interface but without any extensive intermixing in the interfacial regions between the layers, as will be explained in more detail below.
The application is, presently preferably, achieved with a pre-metered coating system which does not introduce any significant shear turbulence at the interface between adjacent dope or polymer solution layers. The present applicants have determined that this absence of significant shear turbulence is in contrast to a post-metered coating system, such as knife coating, which has now been determined to create significant shear turbulence between each of the applied liquid layers, as discussed in the Kools publication. The applicants of the present disclosure believe that they have replicated the Kools"" post-metering process to produce a two layer membrane. The FFBP of the Kools"" membrane resembles that shown in FIG. 7. In light of FIGS. 16b and 16d, this FFBP appears to indicate the presence of a significant asymmetric zone at the interface. The results obtained appear to verify the disclosure as contained in the Kools publication.
This discernable transition layer occurs whether the two dope or liquid polymer layers are applied by two separate casting knifes located some distance apart, as in one experiment, or whether the two casting knifes are built into a single assembly so that there is essentially zero gap between the two polymer solution applications, as disclosed in the Kools"" publication.
The TSE documents cited previously and the Book, xe2x80x9cLiquid Film Coating,xe2x80x9d edited by Stephen F. Kistler and Peter M. Schweizer, Chapman and Hall USA 1997, the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure, lists a number of pre-metered coating processes. These pre-metered coating processes are identified as, but are not limited to, slot coating, extrusion coating, slide coating, and curtain coating. While all of these process are capable of producing multilayer polymer solution coatings without any significant interfacial shear turbulence, it is anticipated that for the production rates typically employed to produce membrane and a typical viscosity of polymer solutions in the 1000 to 5000 cP range, that slot coating process will be preferred.
The concept taught could be applied to nylon, PVDF, PES, PP or any membrane component or polymer capable of producing a phase inversion membrane wherein pore size can be controlled or predetermined through specific control of the polymer solution or dope preparation, which may include formulation of constituents, thermal manipulation, or use of any other pore size controlling steps known to the art prior to the coating process step.
The present disclosure claims, among other innovations, the process of coating multiple layers of polymer solutions, each having been controlled to eventually produce a predetermined pore size when subjected to phase inversion on to a moving nonporous self-releasing substrate with each layer applied with a pre-metering device such as, for example, a slot die and then subjecting such multiple fluid layers to a phase inversion process in, for example, a nonsolvent or solvent/nonsolvent liquid bath in such a manner as to produce an unsupported, multilayer microporous membrane precursor having multiple pore size layers. The moving coating surface material, presently preferably, a nonporous self-releasing support with each layer applied with a pre-metered die is selected so that it is compatible with the dope or polymer solution and will self-release from the wet microporous membrane precursor after the phase inversion process.
It is further contemplated that the individual layers may vary from each other in functional aspects other than pore size, where interlayer mixing must be avoided. Such differences between layers may include polymer end group functionality, polymer composition (use of copolymers), particulate fillers, additives, different molecular weight, wetting characteristics (hydrophilic and hydrophobic), or other functional layer differences, wherein such differences are intrinsic to the individual dopes used to form each of the layers and interlayer mixing must be avoided.
One aspect of the present disclosure includes a process for forming a continuous, unsupported, multilayer phase inversion microporous membrane having at least two distinct symmetrically distributed pore size layers, comprising of the acts of: operatively positioning at least one pre-metering dope applying apparatus capable of applying at least two independently pre-metered polymer dopes relative to a continuously moving nonporous support coating surface; cooperatively applying the pre-metered polymer dopes onto the continuously moving nonporous support coating surface so as to create a multilayer polymer dope coating on the nonporous support coating surface; and subjecting the multilayer dope coating to contact with a phase inversion producing environment so as to form a wet multilayer phase inversion microporous membrane precursor, and then washing and drying this wet precursor structure to form the desired dry multilayer microporous membrane.
Another aspect of the present disclosure includes a process for forming a continuous, unsupported, multilayer phase inversion microporous membrane having at least two layers, comprising of the acts of: operatively positioning at least two pre-metering dope applying or coating apparatus, each capable of independently applying at least one polymer dope, relative to a nonporous support coating surface; sequentially applying polymer dopes from each of the pre-metering dope applying or coating apparatus onto the nonporous support coating surface so as to create a multilayer polymer dope coating on the nonporous support coating surface; and subjecting the sequentially applied polymer dopes to contact with a phase inversion producing environment so as to form a wet multilayer phase inversion microporous membrane precursor, washing and drying said precursor to form the desired dry multilayer microporous membrane.
Still another aspect of the present disclosure includes a multilayer, unsupported, microporous membrane comprising: a first layer having a symmetrically distributed first pore size; and at least a second layer having a symmetrically distributed second pore size, the first and second layers being operatively connected with a distinct change in pore size at the interface thereof such that the multilayer membrane is continuous and does not include any support material.